opEquals on shared object

[Steven Schveighoffer via Digitalmars-d-learn]

On 6/30/16 8:30 AM, jj75607 wrote:
> On Thursday, 30 June 2016 at 12:21:03 UTC, Steven Schveighoffer wrote:
>> On 6/30/16 6:26 AM, jj75607 wrote:
>>> Hello!
>>>
>>> I need to overload opEquals on shared class C
>>>
>>> shared class C
>>> {
>>>     override bool opEquals(Object o) { return false; }
>>> }
>>>
>>> But compilation fails with the message:
>>> Error: function f700.C.opEquals does not override any function, did you
>>> mean to override 'object.Object.opEquals'?
>>>
>>> What am I doing wrong?
>>
>> Object.opEquals is not marked shared. You can't override a non-shared
>> method with a shared one.
>>
>> You need to remove override.
>>
>> But... unfortunately, this may not work in practice. The opEquals
>> handling for objects is pretty much screwed unless you have unshared
>> mutable objects. I think it may work for const objects, but not in a
>> good way.
>>
>> -Steve
>
> Thanks!
>
> But what should I do to fix that code?
>
> shared class C
> {
>     bool opEquals(Object o) { return false; }
> }
>
> class A(T)
> {
>     void f(T a, T b)
>     {
>         if(a == b)
>             writeln("equals");
>         else
>             writeln("non equals");
>     }
> }
>
>
> int main(string[] argv)
> {
>     auto a1 = new A!int;
>     a1.f(1,2);
>
>     auto a2 = new A!(shared(C));
>     shared C c = new shared(C);
>     a2.f(c,c);
>
>
>     return 0;
> }
>
> It fails with
>     Error: none of the overloads of 'opEquals' are callable using
> argument types (shared(C), shared(C))

This is an artifact of the object.opEquals entry point function. It 
handles calling your object's opEquals properly.

Try changing C.opEquals to accept a shared(Object).

If that doesn't work, you may be out of luck until the druntime team 
figures this out. Please file an issue if you can't get it to work.

-Steve